TOF rangefinding with large dynamic range and enhanced background radiation suppression

ABSTRACT

A method for measuring time of flight of radiation comprises emitting modulated radiation in response to a first modulation signal, projecting the modulated radiation onto a scene, receiving radiation, the received radiation comprising a first portion being the modulated radiation reflected by the scene and a second portion being background radiation, converting the received radiation into a signal on a conversion node, the signal on the conversion node having a first and a second signal component, the first signal component being indicative of the background radiation and the second signal component being dependent on the reflected modulated radiation, and determining the time of flight of the radiation based on the second signal component. A corresponding device is also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a continuation of and claims the benefit of thefiling date of patent application Ser. No. 11/171,402, filed Jul. 1,2005.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the field of distance measuring sensorsand methods for time-of-flight (TOF) measurements. More particularly thepresent invention relates to such sensors and methods adapted to achieveefficient background radiation rejection at variable backgroundradiation conditions. The radiation may be visible or infrared light forexample.

BACKGROUND OF THE INVENTION

The basic principle of using time-of-flight (TOF) measurements for rangefinding applications is to measure how long it takes for radiation, e.g.photons to travel over an unknown distance. The unknown distance canthen be deduced from the measured time of flight in combination with theknown speed of the radiation such as light.

Many ways of how to modulate a light source for such TOF measurements,and which strategy to follow for making the distance measurement areknown to a person skilled in the art and are described in patents andscientific literature. Most of these range-finding systems use areceiver in which a mixer is used to demodulate an incoming photocurrentfor finding e.g. a phase or a time period for distance estimation. Thephotocurrent is typically mixed with a reference signal.

There are basically two ways to achieve this mixing. A first way is touse transistors in a multiplier configuration, for mixing thephotocurrent signal with the reference signal. WO 2004/012269 describesa readout circuit using this technique. A light source is pulsed toilluminate a scene, for example comprising one or more objects, and thephase difference between the light reflected from the scene and theoriginal phase of the light source is measured. In order to measure thephase difference, a CMOS photosensor may be used to receive thereflected light and store charge generated during different portions oftime in different storage nodes or pixel cells. The difference betweenthe amount of charge stored in different storage nodes can be used todetermine the phase difference between the original light illuminatingthe scene and the light reflected from the scene. This phase differencecan in turn be used to determine the distance to the scene. This andother transistor mixing methods will be called“transistor-mixing-methods”.

A second way of achieving the mixing is by redirecting photo-generatedminority carriers in the substrate, before they are detected by adiode-junction or by a potential well. WO 98/10255 and WO 99/60629 showsuch methods and corresponding devices for determining the phase and/oramplitude of incident modulated light. By applying a referencemodulation voltage over two photo-gates, the generated minority carriersin the substrate arrive preferentially at one of two detectoraccumulation zones. In WO 98/10255, these accumulation zones arepotential wells, created by a voltage on an adjacent accumulation gate.In WO 99/60629, these accumulation zones are pn-junctions. InEP-03077744.5 co-pending herewith, a bipolar alternative and enhancedversion is described using a reference majority current for redirectingthe photo-generated carriers in the substrate towards detectingjunctions. These second ways of mixing before diode junction detectionwill be called “substrate-mixing-methods”.

Depending on the target specifications, a substrate-mixing-method or atransistor-mixing-method will be preferred.

With both mixing methods it remains a problem to separate signalsoriginating from background light efficiently from signals originatingfrom TOF-light. The background light that is present on an area in ascene of which the distance is to be measured, can be six orders ofmagnitude larger than the light present on this same area andoriginating from the modulated light source. It is known from literatureto reduce this large difference to some extent by using an opticalfilter, which attenuates the visible background light from the TOF lightbased on wavelength differences. In this way a reduction of an order ofmagnitude can be obtained. With a narrow-band optical pass filter andusing a narrow-band laser light source, possibly two orders of magnitudecan be overcome. However, LED light sources are preferred light sourcesfor future TOF range finders, since they may emit Watts of light,whereas lasers may only emit milli-Watts of light in free space foreye-safety reasons.

Further it is difficult to make low-pass filters with a −3 dB frequencyin the 10 Hz-10 kHz range for range finding systems on a small siliconcircuit area, such that each camera pixel can have its own averagingfilter for averaging out the noise after the mixer.

Therefore, no readily usable solution is known for separating signalsoriginating from background light from signals originating fromTOF-light by means of a small circuit while not deteriorating the signalto noise ratio and still achieving 3D camera operation.

AIM OF THE INVENTION

It is an aim of the present invention to improve range finding methodsand systems based on TOF measurements.

It is a further aim of the present invention to obtain improved rangefinding methods and systems based on TOF measurements using small areacircuits supporting 2D-arrays of rangefinders, to thereby provide3D-cameras for many applications.

SUMMARY OF THE INVENTION

The above objectives are accomplished by a method and a device accordingto the present invention.

In a first aspect, the present invention relates to a method formeasuring time of flight of radiation. The method according to thepresent invention comprises

-   emitting modulated radiation in response to a first modulation    signal,-   projecting the modulated radiation onto a scene,-   receiving radiation, the received radiation comprising a first    portion being the modulated radiation reflected by the scene and a    second portion being background radiation,-   converting the received radiation into a signal on a conversion    node, the signal on the conversion node having a first and a second    signal component, the first signal component being dependent on the    reflected modulated radiation and the second signal component being    indicative of the background radiation,-   determining a small signal component of the first signal component,    following the reflected modulated radiation (12), and-   determining the time of flight of the radiation based on the small    signal component of the first signal component.

It is an advantage of the present invention that signals originatingfrom background light can be efficiently separated from signalsoriginating from TOF light.

Converting the received radiation may comprise conversion of thereceived radiation into a radiation induced current signal. A method ofthe present invention may furthermore comprise converting the radiationinduced current signal into the first and the second signal component,the first and the second signal components being voltage signalcomponents. Alternatively, a method according to the present inventionmay furthermore comprise mixing the radiation induced current signalwith a second modulation signal and converting the mixed signal into thefirst and second signal components. In this case the radiation inducedcurrent signal consists of charge carriers flowing in a substrate, andthe mixing of the radiation induced current signal with the secondmodulation signal may be performed in the substrate. The secondmodulation signal may comprise substantially the same modulation as thefirst modulation signal, but is time-shifted.

The first voltage signal component may consist essentially of the smallsignal voltage on the conversion node. The second voltage signalcomponent may consist essentially of the average voltage on theconversion node.

Converting the received radiation into the first and the second signalcomponents may be carried out with a first and a second conversion gainrespectively. The first conversion gain may be increased for decreasingbackground radiation levels. The first conversion gain may be up toorders of magnitude higher than the second conversion gain.

In a second aspect, the present invention provides the use of a methodaccording to the present invention for a distance measurement.

In a third aspect, the present invention provides a device for measuringtime of flight of radiation. The device of the present inventioncomprises

-   a radiation emitting source,-   a modulating device for modulating, in response to a first    modulation signal, radiation emitted by the radiation emitting    source,-   a radiation receiver for receiving radiation, the received radiation    comprising a first portion being the modulated radiation reflected    by a scene and a second portion being background radiation,-   a conversion means for converting received radiation into a signal    on a conversion node, the signal on the conversion node having a    first and a second signal component, the first signal component    being dependent on the reflected modulated radiation and the second    signal component being indicative of the background radiation, and-   a calculation unit for determining the time of flight of the    radiation based on the first signal component.    According to this aspect of the present invention, the conversion    means is adapted for generating a small signal component of the    first signal component, the small signal component following the    reflected modulated radiation. The calculation unit is adapted to    determine the time of flight of the radiation based on the small    signal component of the first signal component.

The conversion means may comprise a photodetector for converting thereceived radiation into a radiation induced current signal. Theconversion means may also comprise amplifying means for amplifying thefirst portion of the received radiation in order to generate the firstsignal component.

A device according to the present invention may furthermore comprise avariable transimpedance converter for converting the radiation inducedcurrent signal into the first and the second signal components, thefirst and the second signal components being voltage signal components.

A device according to the present invention may furthermore comprisemixing means for mixing said radiation induced current signal with asecond modulation signal and a converter means for converting the mixedsignal into the first and second signal components. The secondmodulation signal may comprise substantially the same modulation as thefirst modulation signal, but is time-shifted.

A device according to the present invention, wherein the conversionmeans has a first and a second conversion gain for converting thereceived radiation into the first and the second signal componentsrespectively, may furthermore comprise a gain input terminal forinfluencing the second conversion gain. This is useful for increasingthe second conversion gain for decreasing background radiation levels.The second conversion gain may be up to orders of magnitude higher thanthe first conversion gain.

A device according to the present invention may furthermore comprise alow-pass filter. The low-pass filter may use a MOS-transistor in itstriode region as a means for achieving a large resistance.Alternatively, the low-pass filter may use a MOS-transistor in itssub-threshold regime as a means for achieving a large resistance. Instill an alternative embodiment, the low-pass filter may use a parasiticcurrent leak for achieving a low −3 dB corner frequency of the filter.

In a device according to the present invention, the low-pass filter mayuse a polysilicon-polysilicon capacitor that is at least partiallytransparent for the first portion of received radiation.

An advantage of the present invention can be improvement of the dynamicrange of allowed background illumination on the scene and improvement ofthe dynamic range of the sensed modulated light and hence an improvementin the range of distances that can be measured.

These and other characteristics, features and advantages of the presentinvention will become apparent from the following detailed description,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention. This description isgiven for the sake of example only, without limiting the scope of theinvention. The reference figures quoted below refer to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a prior art TOF measurement system based on transistormixing.

FIG. 2 shows a first preferred embodiment of a method according to thepresent invention valid for both transistor-mixing-methods andsubstrate-mixing-methods at top level, using a demodulator module 35.

FIG. 3 shows a preferred demodulator module 35A based on atransistor-mixing-method with variable transimpedance converter 72A.

FIG. 4 shows a preferred demodulator module 35B based on asubstrate-mixing-method with variable transimpedance converter 72B.

FIGS. 5A and 5B show practical implementations 72C and 72D of thevariable transimpedance converter 72B.

FIG. 6 illustrates the transimpedance gain of the variabletransimpedance converters 72A, 72C and 72D illustrated in FIGS. 3, 5Aand 5B respectively.

FIG. 7 demonstrates that at lower background light (BL) current levels,the transimpedance gain becomes higher.

FIG. 8A is an embodiment of the present invention of a compact low-passfilter with tuneable bandwidth.

FIG. 8B shows the low-pass filter characteristic for various biascurrents.

FIG. 8C shows the preferred domain of operation of the transistor 112 inthe low-pass filter of FIG. 8A.

FIG. 9A illustrates a step of 50 mV on the input 114 of the circuit ofFIG. 8A and its step-response.

FIG. 9B illustrates a step of 200 mV on the input 114 of the circuit ofFIG. 8A and its step-response.

In the different drawings, the same reference figures refer to the sameor analogous elements.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

The present invention will be described with respect to particularembodiments and with reference to certain drawings but the invention isnot limited thereto but only by the claims. The drawings described areonly schematic and are non-limiting. In the drawings, the size of someof the elements may be exaggerated and not drawn on scale forillustrative purposes. The dimensions and the relative dimensions do notcorrespond to actual reductions to practice of the invention.

The description will be given in function of visible light as radiation.However, other kinds of radiation could be used as well, such as forexample infrared or near infrared light since it can be detected by CMOSdetection junctions.

It is to be noticed that the term “comprising”, used in the claims,should not be interpreted as being restricted to the means listedthereafter; it does not exclude other elements or steps. Thus, the scopeof the expression “a device comprising means A and B” should not belimited to devices consisting only of components A and B. It means thatwith respect to the present invention, the most relevant components ofthe device are A and B.

Similarly, it is to be noticed that the term “coupled” should not beinterpreted as being restricted to direct connections only. Thus, thescope of the expression “a device A coupled to a device B” should not belimited to devices or systems wherein an output of device A is directlyconnected to an input of device B. It means that there exists a pathbetween an output of A and an input of B which may be a path includingother devices or means.

Furthermore, the terms first, second, third and the like in thedescription and in the claims, are used for distinguishing betweensimilar elements and not necessarily for describing a sequential orchronological order. It is to be understood that the terms so used areinterchangeable under appropriate circumstances and that the embodimentsof the invention described herein are capable of operation in othersequences than described or illustrated herein.

The invention will be described by a detailed description of severalembodiments of the invention. It is clear that other embodiments of theinvention can be configured according to the knowledge of personsskilled in the art without departing from the true spirit or technicalteaching of the invention, the invention being limited only by the termsof the appended claims. It will be clear for a person skilled in the artthat the present invention is also applicable to similar devices thatcan be configured in any transistor technology, including for example,but not limited thereto, CMOS, BICMOS, Bipolar and SiGe BICMOStechnology.

Furthermore the findings of the present invention are explained withreference to PMOS and NMOS transistors as an example, but the presentinvention includes within its scope a complementary device whereby PMOSand NMOS transistors become NMOS and PMOS, respectively. A skilledperson can make such modifications without departing from the truespirit of the invention.

FIG. 1 shows a conventional way for performing time of flight (TOF)distance measurements for e.g. measuring the distance from a scene 9, bymeans of a transistor-mixing-method.

A light source 15 is provided, which emits light when being driven. Aclock generator 13 generates a first clock signal and puts it on abuffer input node 10, thus driving a buffer 14 that modulates the lightsource 15. The light source 15 can be any light source that has enoughbandwidth to follow the implied modulation, such as for example a LightEmitting Diode (LED) or a Laser, or arrays of light sources that aremodulated simultaneously. By light source 15 emitted modulated light 11reflects partly on the scene 9, thus generating reflected modulatedlight 12 imaged by an optical focussing system such as a lens 16 beforeincidence on a standard pn- or pin-detector 17. In CMOS typically anN-well is used as one terminal of the detector 17 and the substrate asthe other terminal (typically connected to ground). Light impinging onthe detector 17 generates an electrical photocurrent signal 29. Thephotocurrent signal 29 from detector 17 is demodulated by means of afirst mixer or multiplier 18 and a second mixer or multiplier 19. Incase of a transistor-mixing-method, transistors are used in a multiplierconfiguration. For demodulation purposes, clock generator 13 generates asecond clock signal or reference signal on a second node 27 and a thirdclock signal or reference signal on a third node 28, the second andthird clock signals having a 0° and 90° phase relation with respect tothe first clock signal on the buffer input node 10. Demodulation isachieved by mixing or multiplying the output signal 29 from detector 17with the second and third clock signals on nodes 27 and 28 in first andsecond multipliers 18 and 19 respectively. In the present case of atransistor-mixing-method, the obtained photocurrent signal 29 is mixedwith the second and third clock signals by means of transistors. Theoutput signals of the first and second multipliers 18 and 19 are thenlow pass filtered in low pass filters 20 and 21 respectively, and fromthe resulting low pass filtered signals 25 and 26 respectively, acalculating means such as CPU 24 can calculate the phase lag betweenboth low pass filtered signals 25, 26 using for example arc-tangentsfunctions of the ratio of these low pass filtered signals 25, 26. Anoutput signal 22 of the CPU is then a measure for the distance between aTOF measurement device comprising a light source 15 and a detector 17 onthe one hand, and a scene 9 on the other hand.

A 1-MHz clock signal allows measurement of distances unambiguouslybetween 0 m and 150 m, a 10 MHz clock allows measurement unambiguouslyup to 15 m, and a 100 MHz clock allows unambiguous measurements up to1.5 m.

FIG. 2 shows a preferred top-level embodiment of a range finding systemaccording to the present invention for use with any of thetransistor-mixing-methods or the substrate-mixing-methods. The rangefinding system comprises a light source 15 for emitting light 11 onto ascene 9, preferably focussed onto an area of interest, where the lightis reflected, and a demodulator module 35 for receiving reflected light.In order for the light source 15 to emit modulated light, a signalgenerator 30 is provided. This signal generator 30 generates first tofifth clock signals, the second to fifth clock signals having a 0°,180°, 90° and 270° phase relation respectively with the first clocksignal. The second to fifth clock signals are delivered onto second tofifth nodes 37, 38, 39, 40 respectively. A person skilled in the art canalso consider using other or more clock phases in the operation scheme,more clock phases probably leading towards better measurement precisionin exchange for a longer measurement time. Alternatively, instead ofmodulating by means of phases of a clock signal, a person skilled in theart can also consider transmitting a pseudo random bit stream and mixingwith a set of delayed and/or inverted same pseudo random bit streams.The use of pseudo random bit streams, sometimes referred to aspseudo-noise is known in literature by a person skilled in the art. Inthat case, instead of the first and second clock signals it is advisedto use a pseudo random pattern, instead of the third clock signal usethe same pseudo random pattern but bitwise inverted and instead of thefourth clock signal, the same pseudo random pattern but delayed by a bitperiod and instead of the fifth clock signal, the same pseudo randompattern but inverted and delayed by a bit period.

The signal generator 30 also generates a switch-determining signal 33that is determining for a selector 44 to select between the second tofifth clock signals. It is preferred that the selector 44 is switchingsequentially between these four clock signals, thus sequentiallyconnecting a buffer input node 10 of a buffer 14 with second to fifthnodes 37, 38, 39, 40 of the clock generator 30. At each of thesepositions, selector 44 can stay connected for a relaxation period ofe.g. 1 ms. The shorter this relaxation period is chosen, the more data aconnected calculation unit has available for interpretation per unit oftime. Assuming a larger relaxation period assumes having betterintegration capability at the level of the pixel's low-pass filters 71.The buffer 14 drives the light source 15 that emits modulated light 11onto the scene 9, preferably focused on the area of interest. Part ofthis light will be reflected, thus generating reflected light 58. Thisreflected light 58 then arrives on an optical focussing system such as alens 16, through which it is imaged or focussed on the demodulatormodule 35 where the incident fraction is called the reflected modulatedlight (ML) 12. Direct light 57 and indirect light 56, both originatingfrom secondary light sources 43 not intended for the TOF measurement,will impinge on the optical focussing system 16 and thus on thedemodulator module 35 as well. This light incident on demodulator module35 will be called background light (BL) 31. Light sources 43 generatingBL include incandescent lamps, TL-lamps, sunlight, daylight, or whateverlight that is present on the scene 9 and does not emanate from the lightsource 15 for TOF measurement. An aim of the present invention is totake the signal from this BL 31 into account in the subsequent analogsignal processing, thus optimising the distance readout range andprecision.

The first clock signal generated by signal generator 30 on first node 34is preferably permanently oscillating at a basic frequency of e.g. 10MHz, assuming an unambiguous range-finding distance of 15 meter. Otherunambiguous distances use preferably other oscillating frequencies ascan be calculated by a person skilled in the art. The second to fifthclock signals oscillate with the same frequency, but have a 0°, 180°,90° and 270° phase relation with the clock signal on first node 34. Thefirst clock signal on the first node 34 is applied to an input port ofthe demodulator module 35.

FIGS. 3 and 4 illustrate embodiments of demodulator modules 35,indicated as a demodulator module 35A based on atransistor-mixing-method in FIG. 3 and as a demodulator module 35B basedon a substrate-mixing-method in FIG. 4.

ML 12 and BL 31 impinge onto a photodetector 63, 80 internal todemodulator module 35A, 35B and generate, respectively, an ML-current 73and a BL-current 74, which are current responses to the impinging BL 31and ML 12. As already stated earlier, the BL 31 can induce BL-current 74up to 6 orders of magnitude higher than the ML-current 73 induced by theML 12 received for TOF measurements. Demodulator module 35 has mixingmeans to generate the mixing products that are internally low-passfiltered in a low-pass filter 71. The averaged mixing product isavailable in a sequential form synchronised with selector 44 at outputnode 36, where it can be sampled at the end of each relaxation period(e.g. each millisecond) by sample-and-hold circuitry (not represented inthe drawings) and digitised for further digital signal treatment in asubsequent digitising means or calculation unit 150, such as a CPU.

The first clock signal on the first node 34 of the signal generator 30and its duty cycle are expected to be totally independent of theposition of selector 44. When this is the case, the edges in curve 45 ofthe signal on the output node 36 will be only originating from thetransmitted light 11 changing phase each time that selector 44 isswitched to its next position. At the end of each relaxation period, thesignal on output node 36 has to be sampled, as indicated on curve 45 inFIG. 2. During the first relaxation period 47, when by selector 44buffer input node 10 is connected to second node 37, the value at outputnode 36 of demodulator module 35 should converge to a first end value ina relatively short time period, preferably in about half the relaxationperiod (e.g. 0.5 ms). In the subsequent part, e.g. the subsequent halve,of the relaxation period, that value can then be sampled, e.g. at afirst sampling moment 52. During a second relaxation period 48, bufferinput node 10 is connected to third node 38 of the signal generator 30through selector 44. From then on, the light of light source 15 isdriven 180° out of phase with respect to the first clock signal on firstnode 34 of the signal generator 30. The phases 90° and 270° aresubsequently treated similarly in subsequent third and fourth relaxationperiods 49 and 50 respectively. By measuring each second half of therelaxation period, the responses at 0°, 180°, 90° and 270° phase can besampled at first to fourth sampling moments 52, 53, 54, 55 respectively.Dividing the difference between the first two samples taken at the firstand second sampling moments 52 and 53 by the difference of the last twosamples taken at the third and fourth sampling moments 54 and 55, andtaking e.g. the arctangent of the result, a measure for phaseretardation due to the light's TOF is obtained. Using the speed of lightwill deliver the sought distance estimate. When having full 2D-arrays ofthese detectors each having their own circuitry, the full second half ofthe relaxation period can be used for reading out the full array, orpart of the full array in e.g. a sequential form, i.e. pixel after pixelin a row, and row after row.

Having four relaxation periods of 1 ms, distance estimates can becalculated 250 times per second. As aforementioned, shorter or longerrelaxation periods can be used. Very long relaxation periods are howevermore difficult to implement, since as will be shown further on, a filter71 will have to average out over longer periods. The use of very longintegration periods by having a large relaxation-period is further notrecommended since variations of background light intensity may destroythe coherence in the subsequent phase measurements. A relaxation periodbetween 10 ms and 50 ns is therefore advised. However, an attachedcalculating unit such as a CPU can average out the samples of eachphase, or it can average out the difference between pairs of samples, orit can average out the estimated distance on a period as long as wantedfor the given application. More elaborate calculations and estimationscan be implemented as algorithms in the attached calculating unit. Itcan for example check the consistency of subsequent derived phaseestimations, and as such generate a signal that is representative forthe certainty of the achieved distance/phase estimate. The amplitude 51of the small signal voltage on output node 36 of demodulator module 35is proportional to the amplitude of ML-current 73, whereby thetrans-impedance gain is dependent on the BL-current 74.

For obtaining a value related to the average light input power, beingdetermined mainly by the BL 31, a signal relating to an average lightsignal can be made available directly from the demodulator module 35 ataverage light signal node 42. On the other hand, the average voltage 46on output node 36 is depending on this level of the BL 31 as well, andcan thus also be used for obtaining the average light input power or avalue related thereto.

The demodulator module 35 may further comprise a gain node 41, which canbe used for lowering the trans-impedance gain by several orders ofmagnitude when measuring short distances leading to high light levels ofthe ML 12, and thus to high ML-currents 73. A gain input signal at thegain node 41 can be used when the amplitude 51 of the sampled signal onoutput node 36 becomes too large for remaining in a linear domain ofoperation. Optionally, this gain node 41 can be driven by an auto-gaincircuit 59 that will regulate or limit said transimpedance gain based onits input signal on output node 36. This can be organised by having alocal auto-gain system on the circuitry level. However, in twodimensional arrays, it is also an option (not shown in the drawings)that each pixel samples and holds a digital or analog gain valuedelivered to each pixel by the calculating unit. In other words, at themoment when reading out a pixel, the calculating unit may have thecircuit elements to determine the gain for each pixel separately bytransferring to each pixel an analog or digital value that fixes thegain for measurement(s) to follow for that particular pixel. In asimpler implementation, the gain node 41 can be implemented as a commonnode for all pixels for 1D and 2D arrays of range finders.

Further, if implementation of a 1D or 2D array of demodulators 35 isenvisaged, the outputs of all demodulator modules at output nodes 36preferably have to be sampled during the second half of eachrelaxation-period, being, in the example given above, during the secondhalf of each of the first to fourth periods 47, 48, 49, 50. The thusobtained sampled values can then be collected in a frame buffer in theattached calculating unit, e.g. CPU. The signal on the average lightsignal nodes 42 will then deliver a classical grey value image of thescene 9.

In some particular situations it can further be preferred to sample thevalues at output node 36 on first to fourth sampling moments 52, 53, 54,55 (and their subsequent cycles) through extra transistor mixing meanson capacitive voltage nodes that integrate for each phase the averagevoltage value of that phase over a multitude of cycles. This would be atthe expense of extra pixel circuitry present in preferably each pixel.However the required A-to-D conversion rate would be loweredsignificantly and an enhanced signal-to-noise ration can be expected.

FIG. 3 shows a preferred embodiment 35A of a demodulator module 35 incase when a transistor-mixer-method is used. Photodetector 63 can be anytype of conversion element converting light signals impinging on thephotodetector into electrical signals. Preferably photodetector 63 isformed by an N-well in a p-type substrate when using a standard CMOStechnology. In the example given in FIG. 3, photodetector 63 isconnected between a node 66 and ground level. Photodetector 63 receivesreflected modulated light (ML) 12 and background light (BL) 31. Theselight-inputs are converted into photodetector currents, being theML-current 73 and the BL-current 74 signals respectively, flowingthrough node 66, which is also an input to a variable trans-impedanceconverter 72A. Further a gain regulating current source 64 forgenerating a gain regulating current 75 is coupled in parallel to thephotodetector 63. The gain regulating current source 64 can be set by again input signal on gain node 41. The function of the variabletransimpedance converter 72A is of two kinds: on the one hand itconverts the incoming current on node 66 into an average voltage signalon conversion node 65, which is representative for the sum of BL-current74 with the gain regulating current 75; and on the other hand it alsogenerates a small signal voltage on this conversion node 65 that followsthe ML-current 73. Thereby, the small signal transimpedance gain dependson the sum of the BL-current 74 with the gain regulating current 75. Inthe implementation of the variable transimpedance converter 72A this isclearly the case. First and second transistors 60 and 61 are coupled inseries to node 66, each connected as diode, and with higher bias currenttheir transimpedance capability decreases since their transconductanceg_(m) increases. The more diodes like diode 60 and 61 are placed inseries, the larger the transimpedance gain will be. However, there islimited voltage headroom, and only two to three diodes can typically beused for a given power supply voltage and BL 31 range.

At low ML-currents 73, gain regulating current source 64 has to beturned off. Then, the current determining the operation of the variabletransimpedance converter 72A will be the BL-current 74. This currentwill set the bias operating points of node 66 and of a conversion node65. Optionally a transistor 62, coupled between node 66 and the seriesconnection of the first and second transistors 60, 61, can serve as acascode transistor. Conversion node 65 is the node between the cascodetransistor 62 and the second transistor 61. As long as first and secondtransistors 60 and 61 operate in weak-inversion a logarithmic conversionfrom BL-current 74 to the average voltage on node 65 is obtained. Thisweak inversion operation zone is preferred, however, for extreme largeBL-currents 74, transistor 62 and first and second transistors 60 and 61may also operate in their strong inversion regions, as long asconversion node 65 and node 66 do not saturate. The average voltagesignal on conversion node 65 can be low pass filtered by a low-passfilter 76, for bringing out a signal on average light signal node 42that gives information about the level of BL 31.

On top of the average voltage signal on node 65, a small signal 77 (inthe order of 1 μV up to 1000 mV) is present from the ML-current 73. ThisML-current 73 is assumed to be at least an order of magnitude smallerthan the BL-current 74. If that is not the case, gain regulating current75 can be turned on at a small current level. Depending on the magnitudeof the low frequency BL-current 74, the conversion from ML-current 73 toa small-signal voltage on conversion node 65 will be set. This is veryuseful. A high BL-current 74, will deliver a high associated shot-noisecurrent. Shot-noise is the inherent noise that derives from thestochastic nature of the detected photon flux input light, and isproportional the square root of the light intensity. This shot-noisewill be the prevailing noise source in the system as long as aconsiderable amount of BL 31 is present. The higher this shot-noiselevel, the higher the required ML-current 73 must be for reaching agiven signal to noise ration (SNR). To support this, it is useful tohave a dimmed transimpedance conversion between ML-current 73 and thevoltage signal output on conversion node 65.

When there is less BL-current 74, the required ML-current 73 formeasuring at a given SNR will decrease since the shot noise decreases,and low levels of ML-current become useful as well. In this case it isadvantageous to have a higher transimpedance gain, since otherwise theaverage voltage signal 77 will become too small to be further processedwithout adding too much noise from the attached subsequent circuit (herefor example the amplifier 67). So by having a transimpedance gain forthe ML-current 73, that varies with the background light BL 31, the gainat low ML-currents 73 is automatically regulated and dynamic rangeenhanced. When simulating such situations, for example with a Spicesimulator, circuit elements can be designed such that SNR can beacceptable in a wide operational range of BL 31 and ML 12.

At the other side of the dynamic range, when dealing with largeML-currents 73, which is occurring when a scene or objects in a sceneare located at very short distances, or when BL-current 74 is smallerthan ten times the ML-current 73, then current source 64 can be turnedon at a level such that the output oscillation of the average voltagesignal 77 remains small enough and that conversion node 65 doesn'tsaturate. In this way, a large range of amplitudes for ML-current 73become measurable. A range of four orders of magnitude is at leastfeasible, corresponding with a distance variation of a factor of 100.E.g. a measurement range between 10 cm and 10 m can then be supported.

A mixer 69, e.g. a Gilbert Multiplier, which is a multiplier cellcomprising a series connection of an emitter coupled transistor pairwith two cross-coupled emitter coupled transistor pairs, can multiplythe signal coming from the variable transimpedance converter 72A(optionally through an amplifier 67 coupled between the conversion node65 and the mixer 69) with the first clock signal on first node 34. Theresulting signal on node 70, the output node of mixer 69, is thenlow-pass filtered by low pass filter 71, generating the output signal onoutput node 36. For obtaining a signal on the output node 36 like curve45 (FIG. 2) the targeted rise and fall times should be about 35% of therelaxation period to ensure (with some margin) stable date after 50% ofthe relaxation period. In the 1-ms example, rise and fall times of 0.35ms would thus be targeted, meaning that a low-pass filter 71 ispreferred having a −3 dB corner frequency at 1 kHz (as is known by theperson skilled in the art). In FIG. 8A a preferred low-pass filter isdepicted that requires very small semiconductor material area, e.g.silicon area, aiming at a high resolution camera system that can be usedin low pass filter 71.

FIG. 4 shows a preferred embodiment of the present invention when usinga substrate-mixer-method. In this case the BL 31 and the ML 12 areincident on an area of the substrate, whereby the photo-generatedminority carriers are directed towards one or another detectingdiode-junction. The signal on first node 34 is here determining forwhere the photo-generated minority carriers will be collected. FIG. 4can be a representation of a practical way to obtain TOF-measurementsusing a mixer/detector 80, where the photo-generated minority carriersare either diverted or attracted to the cathode of mixer/detector 80(coupled to a node 66), depending on the first clock signal or clockreference signal on the first node 34. Again there will be BL-current 74and ML-current 73. ML-current 73 is this time the current that ispresent due to the modulated light having been mixed with the firstclock signal on the first node 34 in the substrate detector/mixer 80.ML-current 73 component is still originating from to the incidentmodulated light, and carries therefore the same name. In this set-up,the BL-current 74 is also mixed with the reference signal 34. ThisBL-current 74 can be very large with respect to the ML-current 73. Bymixing the BL-current 74 in the detector/mixer 80, a large oscillatingcurrent at the clock signal frequency, e.g. 10 MHz, will be present. Itis therefore advised to place a capacitor 84 between ground level andnode 66, for averaging out this high-frequency component. A gainregulating current 75 may be generated by gain regulating current source64 set by a gain input signal on gain node 41. A variable transimpedanceconverter 72B is used to bias conversion node 65 and average lightsignal node 42 depending on the sum of the average BL-current 74 withthe gain regulating current 75. An optional transistor 62 coupledbetween node 65 and node 66 can serve as a cascode transistor. The smallsignal output on conversion node 65 follows the ML-current 73. Thetransimpedance gain, again being the ratio between the voltage signal 83on the conversion node 65 and the ML-current 73, is determined by themagnitude of the sum of the average BL-current 74 with the gainregulating current 75. When the gain regulating current 75 is turnedoff, again for decreasing BL-current 74, a larger trans-impedance gainfor the ML-current 73 becomes available, being very useful for theincrease in sensitivity that is available due to the lower Shot-Noiseassociated to the lower BL-current 74. For very high inputs of ML 12,the gain regulating current source 64 can be turned on and regulated tolimit the amplitude of the voltage signal 83 at node 65. The latteramplitude can be in the range of 100 μV to 2000 mV. Attached low-passfilter 71 can be used for averaging the remaining signal noise. Again,for relaxation periods of 1 ms, with rise and fall times of 0.35 ms, a 1kHz low-pass 71 is advised. In some cases it can suffice to makeband-pass filter 71 utilising the high impedance of conversion node 65in a simple RC first order filter, whereby only one additional capacitoris required.

Variable transimpedance converter 72B, as shown in this FIG. 4, is inthis example chosen somewhat different from the variable transimpedanceconverter 72A, from FIG. 3. However converter 72A can be used as well inFIG. 4 and converter 72B can be used as well in FIG. 3. Converter 72Bhas the advantage of having additional internal gain. Resistor 86 andcapacitor 87, connected so as to form a low pass filter 89, can bedetermined such that their associated time constant is far below 100 Hz,as can be determined by the person skilled in the art. In that case, thevariations in voltage signal 83 on conversion node 65, would notsignificantly change the voltage on signal node 42. At conversion node65, transistor 85, coupled between the low-pass filter 89 and conversionnode 65, would then be seen as a diode with g_(m)-conductance forBL-currents 74 (assumed slowly varying), and with g_(o)-conductance forfaster oscillating currents, like for the useful frequency component inthe ML-currents 73, typically being around 500 Hz when having arelaxation period of 1 ms. When this is the case, a gain of 20-100 canbe achieved for the small signal leading to an amplified voltage signal83 on conversion node 65. With such an arrangement, the ML-currentsignal 73 is amplified in a first stage, to a voltage signal, with amuch larger amplitude than obtained with a diode construction (like inthe variable transimpedance converter 72A with the series connection oftwo transistors 60 and 61 connected as diodes).

Variable transimpedance converter 72B is somewhat difficult toimplement, since it would require a very large resistor 86 in the 10MΩ-10 GΩ range and/or a very large capacitor 87 in the pF to nF range.FIGS. 5A and 5B show two implementations 72C and 72D for practicalimplementation of the variable transimpedance converter 72B of FIG. 4 ona small area, thus saving substrate space, e.g. silicon space. Capacitor87 is still present, and can be implemented using the gate capacitanceof e.g. a PMOS transistor with its source and drain terminals connectedto supply voltage Vcc. Resistor 86 is now replaced by a resistor circuit91 comprising a transistor 94 that is held preferably in sub-thresholdregime by active gate voltage regulation. The high resistance is derivedfrom using the linear regime of the transistor, sometimes also calledthe triode region of operation, now held at sub-threshold condition. Atransistor 92 together with a current source 93 form a voltage follower,with output node 95 connected to transistor 94 that is serving as saidlarge resistance. A gate source voltage δ over transistor 92 is designedto be small, by having a large W/L aspect ratio of for example 10 fortransistor 92. Current source 93 can be chosen small, e.g. 1-100 nA.Aspect ratio W/L of transistor 94 is preferably small, like ⅕. On FIG.6, curve 111 shows the resulting transimpedance gain between smallsignal input current through input node 66 and small signal voltagesignal on conversion node 65. Current source 93 can be used to determinethe position in frequency of the positive slope in the curve 111shifting the edge left or right, as represented by arrow 114. Curve 110illustrates the transimpedance of the diode solution provided by thetransimpedance converter 72A. All curves on this FIG. 6 are for aBL-current of 1 μA. The distance 113 between curve 110 and 111demonstrates the increase in gain by a factor of 25 by using variabletransimpedance converter 72C of FIG. 5A instead of transimpedanceconverter 72A of FIG. 3.

Variable transimpedance converter 72D, as represented in FIG. 5B, hassimilar gain as transimpedance converter 72C represented in FIG. 5A,both about 8 MΩ, however it has the merit of being very small, which ispreferred when designing a pixel e.g. for a 3D camera, where resolutionand small pixel size are important. The resistor circuit 105 is formedin this implementation by two diodes and a current source 99. The twodiodes, for example implemented by PMOS transistors 97 and 98, areconnected as anti-parallel diodes. Capacitor 87 keeps the gate voltageof transistor 85 to a more or less fixed level at higher frequencies.Current source 99 symbolises the leakage towards the Vcc, due to theattached drain/source diffusion of transistors 97 and 98, at thefemto-ampere level. This system is auto-biasing, the transimpedancebetween input current on node 66 and small signal output voltage onconversion node 65 shown in FIG. 6 by curve 112. The positive edge is atabout 1 Hz, and this frequency can possibly be altered by changing thecurrent of source 99, however this femto-ampere level is difficult tocontrol. Curve 111, obtained by using the implementation oftransimpedance converter 72C as in FIG. 5A, has the advantage thatcurrents up to 100 Hz get attenuated, meaning that BL 31 variations aregetting attenuated more easily as well, however its requiredsemiconductor material area, for example Si-area, for implementation islargest. When it is useful to position the positive edge of curve 111precisely, as indicated by arrow 114, independent of process valueparameters and temperature, the current through current source 93 can bedriven by a replica biasing circuit or regulated through a calibrationprocedure.

Besides the transimpedance converters 72A, 72B, 72C, 72D, as illustratedin FIGS. 3, 4, 5A and 5B, other small circuits can be configured. Auseful option is to insert at position 115 in FIG. 5A a PMOS transistorin cascode configuration. This will increase the gain with a factor 10,in the example given from 8 MΩ to at least 80 MΩ. The person skilled inthe art can work this out further and adapt the transimpedanceconverters 72A, 72B, 72C, 72D to its specifications.

FIG. 7 shows the merit of varying the transimpedance gain of variabletransimpedance converter 72C by changing the level of the BL 31. Curves121, 122 and 123 are for BL-current 74 of 10 μA, 1 μA and 100 nArespectively. It can be seen from the curves that the transimpedancegain increases by a factor of 4-5 per decade decrease in level ofBL-current 74. The prevailing noise factor when having BL 31, is theshot-noise in the BL-current 74 and that is decreasing by a factor of3.2 per decreasing decade of BL 31. These factors nearly match, suchthat the increase in sensitivity at lower levels of BL 31 can easily beexploited.

FIG. 8 shows in more detail how a low-pass filter 89 can be constructedwith a −3 dB frequency that is in the range of 10 Hz to 10 kHz and thatcan be implemented on a small chip area. It is essentially the same asthe low-pass construction formed by circuit 91 with capacitor 87 in FIG.5A. The explanation is somewhat extended here for helping a personskilled in the art to understand the implication and the use of thisfilter more thoroughly.

Compared to a first order low-pass filer based on e.g. an RC filter, afilter of the present invention, as illustrated in an embodiment in FIG.8A, has the feature that the −3 dB frequency is programmable by asettable current. PMOS transistor 106 forms together with current source110 a voltage follower. Output node 109 of the voltage follower followsthe input voltage at input node 107 of the voltage follower, be it witha small voltage δ higher. By keeping W/L of transistor 106 of thevoltage follower high enough, e.g. at least a factor of five, and bykeeping the current of the current source 110 low, the voltage δ overtransistor 106, between gate and source will be below-threshold. PMOStransistor 105 will then be biased most of the time below threshold aswell. Output voltage on output node 108 of the low pass filter, willthen slowly follow the voltage on output node 109 of the voltagefollower. For small signal amplitudes, up to about 50 mV, curves 116,117 and 118 in FIG. 8B demonstrate the low-pass behaviour for currentsof current source 110 with a value of 10 nA, 100 nA and 1 μArespectively. For small signal amplitudes, transistor 105 is always inits linear (also called triode) regime, indicated in FIG. 8C by 119. Toexplain large signal behaviour more in detail, simulations have beenmade when flowing a current of 350 nA through current source 110. Thesmall signal −3 dB frequency is then 2 kHz. With small step-inputs ofe.g. 50 mV on input node 107 of the voltage follower, as illustrated byinput curve 140 in FIG. 9A, the output voltage curve 141 on output node108 of the low pass filter follows with rising and falling edge times ofabout 175 μs. This is consistent with a simple first order linearRC-filter. However, at a large step upwards (illustrated in curve 142),say of 200 mV, applied on input node 107 of the low pass filter, theoutput signal on output node 108 will follow only very slowly, asillustrated by curve 143, visible at edge or slope 144. By a stepdownwards of 200 mV, output node 108 will follow very quickly (edge 145on curve 143), since transistor 105 will see an enlarged voltage betweenoutput node 108, at that moment being its source, and its gate-node 107.Transistor 105 will conduct strongly during the falling edge. In otherwords, the construction in FIG. 8A has a peak-detecting capability forlarger signals, since for falling edges, the output follows relativelyquickly, and for rising edges, the output follows slowly. For smallsignals, below 50 mV, the small signal analysis is valid, and the curves116, 117 and 118 correctly demonstrate the tuneable properties.

This filter can be used like in variable trans-impedance converter 72Cin FIG. 5A (with circuit 91 & capacitor 87), where the non-linear largesignal behaviour is not harmful for good operation of the structure. Theeffects can also easily be simulated, since spice-circuit simulatorswill include large non-linear transient effects with enough precision.

Further, when using this filter as low pass filter 71 in FIG. 3, thefilter can be of great help, since such filter will still middle out thenoise, which has typically a value smaller than 50 mV. The effects ofthe unwanted non-linear peak detecting behaviour for larger signals haveto be limited and taken into account in the design phase. One receivesthe tuneability and the small circuit area in return. Both are veryuseful elements in 3D camera systems where pixel area has to beminimised, and where by choosing the averaging time constant, one canchoose between higher frame rate or better precision.

For implementation of the low-pass filters in the present invention, itis further an option to use capacitors that are partly transparent forthe incident light. A wavelength of choice for the ML 12, is nearinfrared light, with wavelength in the range of 820 nm to 1000 nm. Thisis not visible for the human eye, and can still be absorbed in Silicon.Its penetration depth is about 15 micron, meaning that it can eventuallytravel through thin layers of silicon without showing too much loss.Many CMOS technologies offer the use of poly-poly capacitors, i.e.capacitors between two thin poly-silicon plates. These plates typicallyhave a thickness of 500 nm, such that a poly-poly capacitor can have intotal a thickness of about 1 micron. This will only absorb about 5-7% ofthe incident ML when having a wavelength of 890 nm. By covering a partof the photodetector 63 or modulator/detector 80 with a poly-polycapacitor one can achieve a relative large capacitance for low-passfiltering or averaging purposes.

1. A low-pass filter with an input node and an output node, the filter comprising a voltage follower connected to the input node, the voltage follower comprising a series connection of a current source and an input transistor, such that a voltage across the input transistor provides a controlled voltage drop with respect to an input voltage applied to the input node, and an output transistor connected between the voltage follower and the output node, so that the controlled voltage drop biases the output transistor, wherein the input transistor has a gate connected to the input node, and the output transistor has a gate, and wherein the input and output transistors are coupled with their gates to each other.
 2. A low-pass filter according to claim 1, wherein the −3 dB frequency of the low-pass filter is tuneable by setting the current of the current source.
 3. A low-pass filter according to claim 1, the input transistor having a source, wherein the current source is adapted to deliver a low current so that the voltage over the input transistor between gate and source is below threshold.
 4. A low-pass filter according to claim 1, wherein the input and output transistors are of a same type.
 5. A low-pass filter according to claim 4, wherein the input and output transistors are p-type transistors.
 6. A low-pass filter according to claim 5, the output transistor having a drain and a source, wherein the source of the output transistor is connected to an output node of the voltage follower, and the drain of the output transistor is connected to the output node of the filter.
 7. A low-pass filter according to claim 1, wherein the W/L ratio of the first transistor is adapted to be larger than five.
 8. A low-pass filter according to claim 1, furthermore comprising a capacitor connected between the output node and a power supply.
 9. A low-pass filter with an input node and an output node, the filter comprising: a voltage follower connected to the input node, the voltage follower comprising a series connection of a current source and an input transistor, such that a voltage across the input transistor provides a controlled voltage drop with respect to an input voltage applied to the input node, and an output transistor connected between the voltage follower and the output node, so that the controlled voltage drop biases the output transistor; and a capacitor connected between the output node and a power supply, the filter being for use in a 3D camera system capturing incident light, wherein the capacitor is partly transparent for the incident light.
 10. An image sensing system comprising a low-pass filter with an input node and an output node, the filter comprising a voltage follower connected to the input node, the voltage follower comprising a series connection of a current source and an input transistor, such that a voltage across the input transistor provides a controlled voltage drop with respect to an input voltage applied to the input node, and an output transistor connected between the voltage follower and the output node, so that the controlled voltage drop biases the output transistor.
 11. A low-pass filter with an input node and an output node, the filter comprising a voltage follower connected to the input node, the voltage follower comprising a series connection of a current source and an input transistor, such that a voltage across the input transistor provides a controlled voltage drop with respect to an input voltage applied to the input node, and an output transistor connected between the voltage follower and the output node, so that the controlled voltage drop biases the output transistor, the filter being arranged to operate as a low-pass filter for small signals and a peak detector for large signals. 